i.e) Only when behaviors become typical among an animal population can we conclude that genetic alteration has occurred in that variety of species.

Hopefully this doesn't get too far into formal logic.

So first, translate it: If genetic alteration has occurred in a variety of species, then behaviors have become typical among the population.

g --> b

Negate it: ~(g --> b)

g --> b is equivalent to ~g v b, so we have:

~(~g v b)

Finally, DeMorgan's: g & ~b

So yes, the negation of a conditional is always a conjunction of the antecedent and the negation of the consequent. Which makes sense, because all a conditional is telling you is that it can never be the case that both the antecedent is true and the consequent is false. If you have the negation of that, then it means the antecedent is true, but the consequent is still false.

Oh, so I guess to tie it all back with the original sentence, the negation would be "Genetic alteration has occured in a variety of species, but behaviors have not become typical among the population."

So yes, the negation of a conditional is always a conjunction of the antecedent and the negation of the consequent. Which makes sense, because all a conditional is telling you is that it can never be the case that both the antecedent is true and the consequent is false. If you have the negation of that, then it means the antecedent is true, but the consequent is still false.